Earlier quoted context omitted.
...CodeSandbox has no offline support whatsoever. It sounds like your motivation is to promote an unrelated product in this HN thread, and that's super lame :(
I think they simply didn't understand the distinction that one runs js bins (npm install, webpack build) on a server, and the other in the browser. They also might not understand why it matters.
Essentially CodeSandbox can work offline if you already had the website open while you're online, then you can still continue offline. StackBlitz has the advantage that the tab where your application is can also be opened initially offline, which is handy for showing your application when you have no connection, but if I'm not wrong you cannot continue developing offline. I'd love to hear that I'm wrong, I'm really impressed with StackBlitz.
I'm the author of CodeSandbox btw :).